Commercially available, thermoplastic (hot melt) inks are recognized as a means of producing high quality decorations on glass, glass-ceramic and ceramic articles. Such an ink generally exhibits wax-like characteristics with a low melt viscosity, typically less than 150 poises, at its application temperature. The surface becomes tack-free when cooled to a significant extent below the ink softening (melt) point.
The term "thermoplastic" customarily signifies a material that reversibly softens under the influence of heat. When employed with inks, however, it further signifies melt processibility, that is, the capability of being applied in the molten state to a surface. In some cases, thermoplastic materials may be applied from solution. When so used, they are often termed "lacquers". To avoid ambiguity, thermoplastic inks which are melt processible are often referred to as "hot melt inks".
Thermoplastic inks used in decorating and/or printing generally consist of a vitrifiable, inorganic material dispersed in an organic, flowable vehicle or binder. The inorganic material normally is a clear glass base (flux) containing a pigment and/or an opacifier. Such inorganic materials have been marketed for many years as complete vitrifiable or ceramic colors for use in decorating glass and ceramic ware. The user disperses the inorganic colorant in a vehicle prior to use.
Typically, inks suitable for decorating glass, glass-ceramic, or ceramic surfaces are filled with inorganic material (glass flux, pigments, and opacifiers) in an amount totalling 60-90% by weight. After subsequent application to a surface, such an ink is fired at an elevated temperature, generally in excess of 500.degree. C., to burn out the organics and melt the glass flux. The organics employed must, then, be capable of removal below the firing temperature without affecting the visual appearance and performance of the decoration. Materials employed have included waxes, such as fatty alcohols, polyethylene glycol, and polyethylene glycol esters. The ink is further fortified with a resin that imparts cohesive strength or modifies viscosity, such as ethyl cellulose, hydroxypropyl cellulose, and butyl methacrylate.
Prior thermoplastic inks have commonly been applied by screening methods. These have involved either direct application or offset screening, as disclosed in U.S. Pat. No. 3,688,695. Available ink compositions, as described above, were satisfactory for screening, but not for other techniques. In particular, they were unsuitable for many of the techniques using elastomeric transfer (offset) operations. Such an operation is disclosed in U.S. Pat. No. 3,756,165, and employs oil-based or solvent-based inks.
In glass decorating, a particular problem arose during drying of the oil-based or solvent-based inks to a tacky, semi-solid state upon an elastomer surface prior to transfer onto an article surface. It was observed that an even greater viscosity change was needed to be achieved on the elastomer surface when printing onto a non-porous glassy surface than when printing onto a porous surface, such as unglazed china or wood. The problem was aggravated by the need to use a high level of organic solvent to compensate for the effect of the glass fluxes and pigments on initial ink viscosity. If the organic solvent evaporated slowly, the process was time consuming; it if evaporated rapidly, there were viscosity stability problems. See U.S. Pat. No. 3,756,165, supra.
The viscosity problem could be alleviated with thermoplastic inks, but available formulations lacked pressure sensitivity at temperatures much below their softening points. Thus, it was necessary to heat the inks to temperatures near their softening points during transfer. See U.S. Pat. No. 3,688,695, supra.
An alternative decorating technique involves application of decalcomania transfers (decals) to the surface being decorated followed by firing to permanently affix the decoration to the surface. In this technique, the decoration or design, embodying the inorganic colorant(s), is formed independent of the surface to be decorated, usually on a transfer paper or other carrier. Subsequently, the decal is transferred to the surface, temporarily adhered by an adhesive layer, and then thermally fused onto the surface with burnout of the organics. The three main types of decals are: water-release, heat-release and pressure-release.